Ultrasonic plethysmograph

ABSTRACT

Quadrature phase signals (I and Q) from a pulsed Doppler medical instrument are analyzed to detect phase changes of consecutive echoes from tissue at a depth of interest. Such phase changes are analyzed to determine tissue displacement. Relative displacements at different depths are examined to detect tissue expansion and contraction. The area of the uterus of a pregnant female can be scanned and the frequency of tissue displacement or expansion and contraction examined to determine pulsations due to blood supplied by the fetal heart and/or pulsations due to blood supplied by the maternal heart. Conventional M-mode or B-mode images can be colorized differently for different frequencies of tissue displacement.

This application is a continuation of our copending U.S. applicationSer. No. 07/644,477, filed Jan. 18, 1991, for Ultrasonic Plethysmograph,now U.S. Pat. No. 5,089,498 of U.S. Pat. application Ser. No.07/258,534, filed Oct. 17, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to noninvasive diagnostic medical devicesutilizing ultrasound.

2. Prior Art

In ultrasonic medical devices, ultrasound is beamed into the body by atransmitting transducer and waves reflected from internal matter aredetected by the same or a separate receiving transducer.

A first category of ultrasonic medical devices is the Doppler devicewhich detects the difference in frequency of reflected waves as comparedto the frequency of reflected waves as compared to the frequency of thetransmitted waves. The difference in frequency of a wave before andafter its reflection indicates the velocity of the matter from which thewave was reflected. Doppler devices are often used for analyzing bloodflow through an artery, for example.

In a continuous wave (CW) Doppler device, the ultrasound is continuouslytransmitted into the body and the reflected waves are continuouslymonitored for Doppler shift. Separate transmitting and receivingtransducers are required and it is not possible to differentiate betweenwaves reflected at different distances from the transmitting transducer.

In a pulsed Doppler device, short bursts of ultrasound are transmittedat uniform intervals. Reflected waves from a desired depth can bemonitored by analyzing the received ultrasound signal the appropriatelength of time after a pulse or burst of ultrasound was transmitted.Such a system is referred to as a "range-gated" system. The "range gate"is the short period following the transmission during which the receivedwave form is monitored. In sophisticated systems, there can be severalrange gates so that the reflected wave form is monitored for each ofseveral different short periods following the pulse transmission,corresponding to the reflection of transmitted waves at each of severaldifferent depths. FIG. 1 illustrates the transmitted pulse ofultrasound, the echo and the range gate spaced a predetermined period oftime t_(d) from the beginning of the transmitted pulse. During the rangegate period the echo is sampled or monitored to detect the portionreflected from a desired depth.

In one type of medical Doppler device, the electrical signalrepresenting the reflected wave is demodulated to produce anaudio-frequency signal varying in frequency (pitch) depending on thedifference in detected velocity (Doppler shift). The demodulated signalis fed to a speaker and the resulting sound is indicative of blood flow,such as pulsing of blood through an artery. An example of such a deviceis the Versadopp™10 device available from Diagnostic UltrasoundCorporation of Kirkland, Washington.

In other medical Doppler devices, the signal representing the reflectedwave is fed to a frequency spectrum analyzer which can actuate a videodisplay. Such a display can show a curve representing amplitude on thevertical axis and frequency on the horizontal axis. For analyzing bloodflow, the relative amplitude of each frequency component indicates theproportion of blood flowing at the corresponding velocity. Another typeof display indicates frequency on the vertical axis and time on thehorizontal axis with amplitude represented by brightness. See, forexample, the article titled "VASCULAR ULTRASOUND Ultrasonic Evaluationof the Carotid Bifurcation", by Langlois, Roederer and Strandness,published in Vol. 4, No. 2, of ECHOCARDIOGRAPHY A Review ofCardiovascular Ultrasound (Futura Publishing Company, Inc., 1987).

Medical Doppler devices provide information on the velocity of thematter from which the ultrasound is reflected relative to thetransmitting transducer. Information on the direction, i.e., toward oraway from the transducer, can be obtained by utilizing "quadrature phasesignals" generated by known devices as represented in FIG. 2. Aspertinent to the present invention, the received signal is compared withtwo reference signals that are identical except that one signal is 90degrees out of phase relative to the other signal. Separate demodulatorsproduce outputs commonly known as "I" and "Q" which also are 90 degreesout of phase. Velocity information can be calculated from a singlesignal and velocity and relative direction can be detected from the twosignals. See Peter Atkinson and John P. Woodcock, Doppler Ultrasound andits Use in Clinical Measurement, (San Francisco: Academic Press, 1982),particularly pages 54 to 74.

Another type of noninvasive ultrasonic medical diagnostic device is the"imaging" ultrasound device. Ultrasonic imaging systems are concernedwith the amplitude of the reflected wave as a function of time. Theamplitude and time information indicate the location of interfacesbetween different types of tissue having different impedances toultrasound waves. In a "motion mode" (M mode) system, range gating isutilized to detect amplitude at different depths which can berepresented as the vertical axis on a video display. Brightnessindicates amplitude and the horizontal axis represents time so thatmotion of internal tissue along approximately the line of transmissionand reflection is indicated on the display.

In a brightness mode (B mode) imaging system, echoes are monitored alongclosely adjacent transmission axes such as by sweeping the pulsedtransmitting-receiving transducer. The desired result is atwo-dimensional cross-sectional image of internal structure such as atumour or a fetus. See P. N. T. Wells, Biomedical Ultrasonics, (SanFrancisco: Academic Press, 1977), particularly Chapter 9.

Another pertinent type of medical diagnostic device is theplethysmograph designed to measure expansion and contraction of tissuecaused by pulsation of blood through the tissue. Some such devicesutilize a fluid-filled cuff encircling a limb and measuring displacementof fluid in the cuff (water-filled) or change in pressure (air-filled).Other such devices measure change in tissue electrical impedance orstretching of tissue (strain gauge) or change in optical properties ofthe tissue being examined (fluctuations in translucency).

There have been attempts to measure displacement of internal matter byuse of medical ultrasound devices, primarily movement of arterial walls,such as described in: Charles F. Olsen, "Doppler Ultrasound: A Techniquefor Obtaining Arterial Wall Motion Parameters," IEEE Transactions; A. P.G. Hoeks, "On the Development of a Multigate Pulsed Doppler System WithSerial Data Processing." Ph.D Thesis University of Limburg, Maastricht,Holland, 1982; and Craig J. Hartley et. al., "Doppler Measurement ofMyocardial Thickening With a Single Epicardial Transducer," AmericanJournal of Physiology, 245: 6 (1983), pages H1066-H1072. In general,Doppler techniques are utilized to determine tissue velocity. Velocityas a function of time indicates distance or displacement. An accuratemeasurement is difficult because of the errors introduced duringdetection of frequency and the mathematical frequency analysis. Morepertinent to the present invention is the system described in L. S.Wilson and D. E. Robinson, "Ultrasonic Measurement of SmallDisplacements and Deformations of Tissue," Ultrasonic Imaginq, 4 (1982),pages 71 to 82, because in that system there is an attempt to monitorthe phase of the reflected ultrasound. to the devices measuringfrequency shift, the phase signal is analyzed mathematically byintegration to approximate displacement of tissue. Consequently, errorscan be introduced both in phase measurement and.

SUMMARY OF THE INVENTION

The present invention provides an ultrasonic medical diagnostic devicecapable of analyzing quadrature phase signals I and Q in a pulsedtransmission range gated system to measure the phase change of the echoat different depths. The phase information is analyzed over time todetermine directly displacement of the tissue that reflected theultrasound. The phase information can be converted to binary numbers forstorage in the memory of and analysis by a computer. The diagnosticdevice quantifies the displacement of tissue at different depths andcompares the displacement at difference depths for quantifying therelative expansion and contraction of tissue caused by blood passingthrough it. An output signal can indicate frequency and/or amplitude oftissue expansion and contraction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a pulse of ultrasoundtransmitted into a patient's body by an ultrasonic by internal bodymatter toward such device and the time interval for sampling echo todetermine the signal reflected from a particular depth of interest.

FIG. 2 is a schematic block circuit diagram of components of a knownultrasound Doppler device.

FIG. 3 is a block circuit diagram of an ultrasonic medical diagnosticdevice in accordance with the present invention.

FIGS. 4, 5, 6 and 7 are corresponding diagrams and graphs of signals ofthe type analyzed by use of the ultrasonic medical device in accordancewith the present invention to obtain information on the phase ofultrasound echoes.

FIG. 8 is a chart showing two-bit binary representations of change inphase of returning echoes and indicating the significance of the phasechanges or transitions.

FIG. 9 (on the drawing sheet with FIG. 3) is a very diagrammatic blockcircuit diagram of a medical ultrasonic diagnostic device in accordancewith the present invention.

FIG. 10 and FIG. 11 are very diagrammatic block circuit diagrams ofmodified devices in accordance with the present invention.

DETAILED DESCRIPTION

With reference to FIG. 3, the medical ultrasound device in accordancewith the present invention utilizes a conventional pulsed transceiver 1for transmitting bursts of ultrasound into the body. The transceiveralso detects reflected ultrasound and converts the reflected signal intoa corresponding electrical signal. Such electrical signal is fed to aconventional demodulator component 2. The demodulator componentpreferably provides the standard "I" and "Q" signals. In accordance withthe present invention, the demodulated signal is fed to a phase detector3 which, for each transmitted pulse, determines the phase of thereturning echo at each of several range gates, i.e., the phases forultrasound reflected at each of several different depths. As discussedin more detail below, the difference in phase from one pulse to the nextcan provide a very precise indication of the distance traveled by thereflective tissue. Phase detector 3 provides a digital output indicativeof the phases of the monitored reflected signals from different depthsand feeds the digital information to a computer 4 for storage in memoryand analysis prior to actuation of an output unit 5. In some cases theoutput unit will generate an audio output and in other cases the outputunit will generate a video output.

In one embodiment, the quadrature signals I and Q are obtained by use ofa model 400B pulsed Doppler unit available from Advanced TechnologyLaboratories. In such unit, the I and Q signals are normally fed to asample and hold gate and filtered to give the familiar audio outputassociated with Doppler. For use in the present invention, the I and Qsignals are obtained prior to such sample and hold gate. Thetransmission frequency of such unit is 5 megahertz and the pulserepetition frequency can be modified to be an integral multiple of 480hertz up to 3.85 kilohertz depending on the time interval between pulsesrequired to extract information from the maximum depth of interest. Eachpulse can consist of approximately five cycles. Following each transmitpulse, the returning echo can be sampled up to sixty-four times atintervals of 1.6 microseconds. Assuming a speed of ultrasound throughbody tissue of 154,000 centimeters per second, the 1.6 microseconds timeinterval between samples corresponds to sampling reflections occurringabout every 1.25 millimeters from the face of the transducer up to amaximum depth of about 8 centimeters. In the representative embodiment,phase shifts of plus or minus 90 degrees are measured from sample tosample. For the 5 megahertz transmission frequency, a 90-degree changein phase corresponds to movement of the tissue reflecting the ultrasoundof about 40 micrometers toward or away from the transducer.Consequently, unless the reflecting tissue is displaced more than 40micrometers in the time interval between consecutive pulses, the digitaloutput from the phase detector provided a heretofore unattainablyprecise measurement of the displacement of tissue at each of severalclosely spaced depths.

More specifically, in the representative embodiment and I and Q signalsfrom the model 400B ATL unit have a 6-volt DC offset. Minus 6 volts isintroduced to remove the DC component. The remaining AC quadraturesignals I and Q are sampled simultaneously but independently and thesign (positive or negative) of each signal is detected. If the voltageis positive, that signal is assigned a value of "1" and if the voltageis negative the signal is assigned a value of "0". There are fourpossible combinations for the I and Q signals as represented in FIGS. 4through 7. The diagrams at the left indicate the phase quadrant of thetwo signals at the time of sampling. The graphs at the right representthe corresponding positions of the signals at the time of sampling whichis indicated by the arrows.

Referring to FIG. 4, during the "first" 90 degrees, both I and Q arepositive. Q is arbitrarily assigned as the right digit of a two-bitbinary number and I as the left digit. The FIG. 4 relationship ischaracterized as "11".

Referring to FIG. 5, during the next 90 degrees of phase (90 degrees to180 degrees), signal Q will be positive but signal I is negative so tatthe relationship is "01".

With reference to FIG. 6, during the next 90 degrees of phase (180degrees to 270 degrees), both signals are negative and the relationshipis assigned the binary number "00".

Finally, as represented in FIG. 7, during the next 90 degrees of phase(270 degrees to 360 degrees) signal I is positive and signal Q isnegative and the relationship is assigned the value "10".

At each sampling, the appropriate two-digit binary number is stored incomputer memory. In order to conserve memory space, such number can bestored as two bits of a longer number having other two-bit combinationsrepresenting the sampling at different depths.

At the end of the appropriate period, a new pulse is transmitted and asecond sampling is taken for each of the discrete depths. The previoussampling is referred to as "last line" (LL) and the current sampling isreferred to as "this line" (TL). FIG. 8 illustrates the possible changesfrom "last line" to "this line" and their significance. For example, ifthe previous sampling for tissue at a given depth was "00" and the newsampling indicates the same value, no displacement has occurred, whereasif the new value is 01, there has been displacement of 40 micrometerstoward the transducer. If the "this line" value has changed to 10, therehas been 40 micrometers displacement away from the transducer. It willbe seen that there are four possible transitions (00 to 11, 01 to 10, 10to 01, 11 to 00) which are ambiguous. Those transitions arecharacterized as "aliased data" and ignored. In practice, however,aliased data rarely occurs because of the precision of the unit and thefast pulse repetition frequency.

In summary, with reference to FIG. 9, the I and Q quadrature signalsfrom the ATL/400B pulsed Doppler are shifted to remove the DC component,as indicated by boxes 7, and detectors 8 in combination with a binarynumber generator circuit 9 characterize the phase of the echo signal in90-degree increments by means of a two-digit binary number. The computer4 stores the "last line" and "this line" data and computes the amount ofdisplacement at the indicated depth. At the next sampling, the previous"last line" data is replaced by the previous "this line" data. Updated"this line" data is used to calculate the next displacement.

The very precise determination of tissue displacement at discrete butclosely adjacent depths makes possible new noninvasive diagnosticultrasonic medical devices. One such new instrument allows detection ofamplified audio frequency vibrations occurring in tissue inside thebody. The beating of the heart, for example, induces audible frequencyvibrations of internal tissue, some of which can be heard at the surfaceby use of a stethoscope. With reference to FIG. 10, the computer 4calculating the small, rapidly occurring displacements of internaltissue can be programmed to concentrate on a depth of interest anddetect displacement oscillations or pulsations (echo phase reversals)occurring at such depth at a frequency in the audible frequency range.The computer can actuate an output unit 5 to generate a correspondingaudio output at the detected frequency. Amplitude can be indicated byvolume. The overall effect of such device is similar to placing thesensing end of a conventional stethoscope inside the body where soundscan be "heard" which otherwise would be muted in transmission to thesurface or which may be drowned out by background or other noises. In aspecialized application, the depth (range gate) of interest can beselected to overlap fetal tissue pulsating due to pounding of the fetalheart so that the output would consist primarily of fetal heart toneswhich can be difficult to hear by use of a conventional stethoscope.

In a modified device, the computer 4 can be programmed to calculate thefrequency spectrum or a separate frequency spectrum analyzer 10 can beused for the audio frequency output. The frequency spectrum analyzer 10can trigger a video display unit 11. Such display unit can indicate thedominant frequency of tissue vibrations sensed as a function of time.Such information could be useful to analyze development of the fetalheart, for example.

As indicated in broken lines in FIG. 10, in a related device the signalfrom the ultrasound transceiver can be fed to a substantiallyconventional M mode imaging unit to provide a familiar video outputdisplay shown in FIG. 3 is. There are known M mode units which generateor accept audible frequency Doppler information to colorize the displaydifferently depending on the velocity detected at different depths. Inaccordance with the present invention, rather than colorizing the M modedisplay as a function of velocity, the output of the frequency spectrumanalyzer 10 can be fed to the M mode imaging unit to colorize the M modedisplay depending on the audible frequency vibrations detected in tissuedisplacement. The depth of detection of the audio frequency displacementwould be indicated in the M mode display, colored differently fordifferent frequency ranges.

With reference to FIG. 11, in a modified system, the computer 4 can beprogrammed to detect, calculate and quantify other characteristics ofthe underlying tissue or matter being scanned. For an "ultrasonicplethysmograph" the computer 4 is programmed not only to quantify thedisplacement of tissue at different depths, but also to compare thedisplacement occurring at different depths and quantify the relativeexpansion or contraction of tissue. Because of the multiplicity of rangegates, plethysmography information from a desired depth can be obtained.Prior plethysmographs may allow approximate evaluation of gross volumechanges such as the cross-sectional size of an entire limb or evaluationof surface changes such as fluctuations in translucency due to pulsingof blood through the tissue. By use of the present invention, the phasedetector allows computation of small displacements at different depthswhich can be compared to reveal expansion and contraction of a sectionof tissue deep beneath the skin.

The frequency of pulsation of the tissue can be calculated at anydesired depth of interest, as well as the amplitude of pulsation.Evaluation may reveal or rule out irregular blood supply to the region.Similarly, at a desired depth of interest the time delay from thebeginning of the cardiac R wave to expansion of underlying tissuerepresenting the surge in blood supply may be useful for detectingirregularities. As for the ultrasonic stethoscope discussed above, thefrequency spectrum of tissue expansion and contraction can be calculatedand displayed by use of a frequency spectrum analyzer 10. In a moresophisticated instrument, a two-dimensional B mode imager 13 can be fedthe frequency spectrum information to colorize the two-dimensional,cross-sectional display and provide new information or make it easier tointerpret. For example, one popular use for B mode imaging systems is todisplay representations of a fetus inside the uterus. The presentinvention can detect pulsations of tissue indicative of tissue suppliedby the maternal heart which, at rest, would be beating at less than 100beats per minute and colorize those portions of the B mode image. Whereexpansion and contraction of tissue at a frequency much higher than 100beats per minute is detected, the corresponding portions of the B-modeimage can be colorized differently to indicate tissue supplied by thefetal heart.

Apart from colorization of B mode images, the information on tissueexpansion and contraction can be useful for medical diagnosis. Tissuesdetected to have a delayed expansion (compared to the normal delay froma cardiac R wave, for example) may indicate plugged blood supplyvessels, and tissues having accentuated expansion amplitude in a suspectarea, such as the breast, may indicate irregular increased blood supplyto a specific area such as a tumor.

We claim:
 1. The method of quantifying relative displacement of internalbody matter at different depths below the skin which method comprisestransmitting a first burst of ultrasound into the body from the skin,measuring the phase of ultrasound of the first burst reflected frominternal body matter a first predetermined period after transmission ofthe first burst, measuring the phase of ultrasound reflected by internalbody matter a second predetermined period after transmission of saidfirst burst, subsequently transmitting a second burst of ultrasound intothe body, measuring the phase of ultrasound of the second burstreflected from internal body matter a predetermined period of the sameduration as the first predetermined period after transmission of thesecond burst, measuring the phase of the ultrasound of the second burstreflected from an internal body matter a predetermined period of thesame duration as the second predetermined period after transmission ofthe second burst, and comparing the change in measured phases of thereflected ultrasound to determine the relative displacements of bodymatter at the different depths corresponding to the length of timerequired for ultrasound to travel to and return from body matter duringthe first and second predetermined periods.
 2. The method of determiningand registering variations of expansion and contraction of body tissueby blood passing through the tissue which comprises scanning such tissuewith a pulped Doppler medical instrument by transmitting waves towardsuch tissue and detecting and analyzing echoes of such waves, measuringdisplacement of such tissue at a plurality of depths of interest bymeasurement of phase change of the echoes, comparing such displacementsat different depths so as to detect relative expansion and contractionof the tissue, and providing an output indicative of expansion andcontraction of such tissue.
 3. The method defined in claim 2 includingproviding an output of the frequency of expansion and contraction of thetissue.
 4. The method defined in claim 2 including providing an outputof the amplitude of expansion and contraction of the tissue.
 5. Themethod defined in claim 2 including measuring the time delay from thebeginning of a cardiac signal to the beginning of expansion of thetissue signifying a surge in blood supply through the tissue.
 6. Themethod defined in claim 2 including providing an output of the frequencyspectrum of expansion and contraction of the tissue.
 7. The methoddefined in claim 2, including generating a video display of an image ofthe tissue and colorizing the display based on the output indicative oftissue expansion and contraction.
 8. The method of quantifying expansionand contraction of internal body tissue in a region below the skin toevaluate blood supply to the tissue which method comprises transmittinga first burst of ultrasound into the body from the skin, measuring thephase of ultrasound of the first burst reflected from internal bodymatter a first predetermined period after transmission of the firstburst, measuring the phase of ultrasound reflected by internal bodymatter a second predetermined period after transmission of said firstburst, subsequently transmitting a second burst of ultrasound into thebody, measuring the phase of ultrasound of the second burst reflectedfrom internal body matter a predetermined period of the same duration asthe first predetermined period after transmission of the second burst,measuring the phase of the ultrasound of the second burst reflected frominternal body matter a predetermined period of the same duration as thesecond predetermined period after transmission of the second burst, andcomparing the change in measured phases of the reflected ultrasound todetermine the expansion and contraction of body tissue in the region ata depth corresponding to the length of time required for ultrasound totravel to and return from body matter during the first and secondpredetermined periods.
 9. The method of determining and registeringvariations of expansion and contraction of internal body tissue toevaluate blood supply to tissue which comprises scanning such tissuewith a pulsed Doppler medical instrument by transmitting waves towardsuch tissue and detecting and analyzing echoes of such waves, measuringdisplacement of such tissue at a plurality of depths of interest bymeasurement of phase change of the echoes, comparing such displacementsat different depths so as to detect relative expansion and contractionof the tissue, and providing an output indicative of expansion andcontraction of such tissue.
 10. The method defined in claim 9, includingproviding an output of the frequency of expansion and contraction of thetissue.
 11. The method defined in claim 9, including providing an outputof the amplitude of expansion and contraction of the tissue.
 12. Themethod defined in claim 9, including measuring the time delay from thebeginning of a cardiac signal to the beginning of expansion of thetissue signifying a surge in blood supply through the tissue.
 13. Themethod defined in claim 9, including providing an output of thefrequency spectrum of expansion and contraction of the tissue.
 14. Themethod defined in claim 9, including generating a video display of animage of the tissue and colorizing the display based on the outputindicative of tissue expansion and contraction.